The present invention relates to a chain welding machine for the electric resistance pressure butt welding of prebent, C-shaped links of a length of chain by means of two upsetting tools which sieze the chain link to be welded by its two rounded ends. The tools move at least relatively toward each other in opposite directions and with a drive train or loop which begins at a spring loaded cam gear and ends at a movable upsetting tool and includes a hydraulic cylinder as one of its gear elements.
In the case of a known chain welding machine of this general type disclosed in German Pat. No. 2,317,691, corresponding to U.S. Pat. No. 3,906,183, which, however, employs a fishplate as a rigid gear element instead of a hydraulic cylinder, the cam disk of the spring loaded cam gear is formed in such a way that, for a standard chain link to be welded, an ideal timing sequence is provided for the movement of the two upsetting tools before and during the welding. However, since the chain links of an unwelded length of chain are subject to flunctuations with regard to their accuracy of size and of shape due to bending errors and tolerances of the dimensions and ultimate stress values, more or less significant deviations are obtained during practical operation from this ideal timing sequence of the motion. After the preliminary stroke, preferably effected at a rapid rate, of the upsetting tools at the end of which the upsetting tools contact the two rounded ends of the chain link to be welded, there follows the first portion of the upsetting stroke, which can be called the joining stroke, at the end of which the ends of the wires of the chain link to be welded, which were still separated from each other after the preliminary stroke by a more or less large gap, contact each other with a joining pressure depending upon, for example, the length of the chain link and differing from the joining pressure determined for the standard chain link by being larger or smaller than the normal joining pressure to an extent as determined by the magnitude of the difference between chain link to chain link. Since lateral dislocations of the ends of the wires to be welded to form a welded chain link, i.e., misalignments, are not taken into consideration in this instance, the surface across which the ends of the wires to be welded will touch each other can be considered as being constant so that the term "joining pressure" can be replaced by the term "joining force."
The flunctuation of the actual joining pressure within an uncertain fluctuation width containing the normal joining pressure results in an undesired fluctuation of the electrical resistance which prevails at the point of the current passage from the one end of the wire to the other end of the wire of the chain link to be welded. The resistance variations, in their turn, lead to fluctuations in the intensity of the welding current and these again to fluctuations of the welding temperature obtained at the end of the constant welding time. However, if the welding temperature differs from chain link to chain link, then, as has been proven in practical operations, there cannot be obtained a consistently good weld and, consequently, a constantly high quality of the welded chain links. Therefore, in view of the dependence of the welding temperature on the joining pressure which the ends of the wires of the chain link to be welded exert on each other, it is of decisive importance that the joining pressure is adapted to the standard joining pressure and that it is maintained at a constant value from one chain link to another.
Therefore, the chain welding machine known, for example, from German Pat. No. 2,457,180 and being of the type mentioned above, has the task of providing a machine which, despite the irregularities in the characteristics of the chain links occurring from chain link to chain link of a length of chain, i.e, despite fluctuations, particularly, in the length of the link and in the width of the gap, is capable of manufacturing chains with uniformly welded and, therefore, high-quality chain links. In order to resolve this problem, German Pat. No. 2,457,180 proceeds from the general idea of favorably influencing the joining pressure with which the opposite wire ends of the chain link to be welded contact each other after the upsetting tools have effected their preliminary and joining strokes.
In German Pat. No. 2,457,180, the problem is solved by means of a buffer element variable in length in the train which, independently of its length, transfers an adjustable maximum pressure between the two adjacent gear elements in its longitudinal direction. This buffer element is the hydraulic cylinder mentioned in the introduction by means of which it is achieved that, when a minimum force is made available by the cam gear, a constant joining pressure prevails which is exerted by the two wire ends of the chain link to be welded upon each other independently of any abnormal characteristics the chain link to be welded may have, apart from misalignments. Therefore, the compensation of the path achieved by the hydraulic cylinder produces constantly equal initial conditions for the electrical resistance pressure butt welding so that chain links can be made which have uniformly good welds. Even if the compensation of the path accounts only for some tenths of a millimeter, it prevents large pressure differences which would have a detrimental effect for the actual welding. Another advantage of the hydraulic cylinder as a buffer element consists of the fact that even the heat expansion of the upsetting tools in the upsetting direction, produced by the previous weldings, can no longer influence the joining pressure with which the wire ends contact each other because compensation is also effected for such expansion.
Hydraulic butt welding machines are also known. However, they do not operate mechanically, in the narrow sense of this, but hydraulically so that the flow of power is essentially bound to the used liquid pressure medium while, in the case of the German Pat. No. 2,457,180, the flow of power passes through rigid machine parts and only a single element in the open train for the motion of the upsetting tools is temporarily non-rigid which, however, behaves as a rigid machine part during the upsetting after the joining of the wire ends of a chain link to be welded, and particularly during the final upsetting process which is characterized by high pressure so that there is no mechanical difference from the chain welding machine known from German Pat. No. 2,317,691.
On the basis of the machine mentioned above as known from German Pat. No. 2,457,180, it has been shown that new disadvantages occur owing to the insertion of a hydraulic cylinder as a buffer element, the cylinder and piston rod of which are connected to the two adjacent gear elements of the drive train. In the case of the known machine, the stroke volume of the hydraulic cylinder is actually connected through a piping with a pressure reservoir acting as a hydropneumatic spring which customarily consists of a casing of constant shape with a variable volume bellows arranged in the casing inside of which there prevails an almost constant liquid pressure and outside of which there prevails an essentially constant gas pressure which can be adjusted for the initial spring tension. The adjustment of the initial tension pressure is, in this instance, effected by changing the amount of the fluid or, preferably, the amount of the gas for which purpose auxiliary devices and measuring instruments are required. For their operation, actually for the adjustment of the joining pressure with which the opposite wire ends of the chain link to be welded are to contact each other, a period of time is required which is comparatively long. Additionally, it must be taken into consideration that the desired joining pressure is always slightly exceeded when approaching this pressure, despite the desired flat characteristic spring line of the pressure reservoir so that the excess to be expected must be considered when setting the desired joining pressure and which can only be done in a rough manner.